Crystal Structure Analysis by Diffraction

Abstract

Electrons are Bragg diffracted at lattice planes. The geometry of a diffraction pattern can be described by the kinematical theory. For the discussion of intensities it is necessary to use the dynamical theory of electron diffraction and the Bloch-wave model. Because a Bloch-wave field has nodes and antinodes at the nuclei and the probability density at the nuclei depends sensitively on the tilt relative to the Bragg position, the backscattering coefficient shows an anisotropy. When an electron probe is rocked, this anisotropy of the backscattering results in the electron channelling pattern (ECP). For a stationary electron probe, the angular distribution of backscattered electrons is modulated by excess and defect Kikuchi bands, leading to the formation of an electron backscattering pattern (EBSP), which can be observed on a fluorescent screen beside the specimen and recorded on a photographic emulsion or via a TV camera. At oblique incidence of the electron beam, the reflection high-energy electron diffraction (RHEED) pattern may contain Bragg diffraction spots and Kikuchi lines. ECP and EBSP are related by the theorem of reciprocity. These patterns contain information about the crystal structure, orientation and distortion.